Vane-type gas flow control valves are known. In particular, U.S. Pat. Nos. 2,435,092, 2,443,263 and 2,435,091, all to H. A. Meyer, show pie-shaped vanes radially supported to individually rotate on an axis slightly below the approximate vane center line. A similar control valve is shown in U.S. Pat. No. 4,187,879 to Fermer et al.
In the prior art, radial vanes supported in a flange usually penetrate the walls of the flange for vane control purposes. Where pressure inside and outside of the flange is generally equal this presents no problem. However, where the inside and outside pressure is drastically unequal, as in vacuum systems, wall penetration of support vanes is a problem, since the penetration zones create gas leak zones. Without wall penetration the vanes cannot be readily controlled or supported.
Very low-pressure vacuum chambers are used to perform such processes as radio frequency or d.c. sputter deposition, plasma etching, low-pressure chemical vapor deposition and reactive ion etching. The process vacuum chamber must be evacuated to pressures on the order of 1.times.10.sup.-6 Torr as quickly as possible to reduce overall process time. It is then necessary to gradually introduce into the process chamber a gas, usually inert, to displace the remaining air molecules. The gas is ionized by a cathode and provides a plasma source to perform a variety of processes. The processes generally require that the chamber maintain a fixed pressure, say 1.times.10.sup.-1 Torr for plasma stability.
Gas throttling valves are used in such vacuum systems to maintain the desired chamber pressure by controlling the effective speed of pumping of the process chamber. At present there are two standard methods of controlling process pressures. One method, the "upstream method," requires that a throttling valve, located between the process chamber and the vacuum pump be partially closed to an accurate pre-determined position. Then, process gas is slowly introduced into the chamber by a servo controlled inlet valve to attain the proper pressure. A transducer measures the process chamber pressure and feeds an electrical signal to a controller which adjusts the opening of the servo controlled valve, thereby maintaining the proper pressure. Such a throttle valve has a pneumatic actuator to provide the open and close functions of the valve, and a micrometer barrell which provides an accurate and adjustable abutment or stop, to place the valve in the proper position for restricting effective pumping speed.
In a second method, the "downstream method," requires that the pneumatic actuator and micrometer assembly on the throttling valve be replaced by a servo drive motor directly coupled to a shaft. The valve may then be modulated by electrical signals sent to it from the servo controller. A gas inlet valve is opened to a fixed position and process pressure is maintained downstream of the chamber by effective modulation of the throttling valve.
Prior art throttling valves have used iris-type vanes and semiphore shutter-type vanes. One of the problems with such vanes is that sometimes the interior mounting of the vanes, within the inner periphery of a flange, baffles the flange aperture so that the full aperture is not available when the vanes are fully open.